1. Field of the Invention
This invention relates to a method for decontaminating radioactive metal surfaces with an aqueous solution containing formic acid.
2. Description of Prior Art
Several different methods are known for decontaminating radioactive metal surfaces. The use of fluoroboric acid to decontaminate radioactively contaminated surfaces is taught by U.S. Pat. No. 5,008,044. The method taught by the '044 patent is suited for decontamination of surfaces comprising metallic as well as mineral substances. The advantage of the method taught by the '044 patent is the high absorbency of the decontamination agent used, which provides a great stripping depth, making the method particularly suitable for cleaning medium and severely radioactively contaminated items of various materials. Appropriately, the method taught by the '044 patent is also used in decontamination efforts at Chernobyl, Russia. The high metallic content permits electrolytic regeneration of the metals. Decontamination of tanks is costly, however, and produces a large amount of waste because of the acid residue present. The toxicity of the decontamination agent poses an additional problem, particularly at higher temperatures, such as above 130.degree. C., when the decontamination agent pyrolizes into toxic borofluoride.
Another decontamination method, taught by U.S. Pat. No. 4,508,641, uses formic acid and/or acetic acid as a decontamination agent and at least one reducing agent, such as formaldehyde and/or acetaldehyde. The '641 patent teaches a method for decontaminating reactor cooling coils, with which steel surfaces can be cleaned with relatively small quantities of chemicals and rinsing water, and wherein used decontamination solution is reprocessed. The addition of reducing agents causes the iron ions to remain stable in the solution, prohibiting the formation of compounds. In a system with closed loops, prohibiting the formation of compounds is crucial for preventing the formation of sediment from settling compounds. The iron compounds are only separated from the decontamination solution in a second step of the decontamination method taught by the '641 patent. Because the entire decontamination process takes place in a closed loop, either the decontamination agent must be continuously injected because it is stoichiometrically depleted, or high concentrations of the acids must be used. On the other hand, the decontamination of a tank does not present such problems. However, cleaning and decontaminating the entire cooling medium in a closed loop according to the decontamination method of the '641 patent is extremely problematic because of the formaldehyde that is present as a reducing agent. A complete decontamination below the permissible threshold of 0.37 Bq/cm, for example, is hardly possible. Nevertheless such complete decontamination of the entire cooling medium is not required inside the cooling loops of reactors.